In recent years, a technique by which a thin film transistor (TFT) is manufactured using a semiconductor thin film (having a thickness of about several nanometers to several hundred nanometers) formed over a substrate having an insulating surface has attracted attention. Thin film transistors have been widely applied to electronic devices such as ICs and electro-optical devices, and their development especially as switching elements for an image display device has been accelerated. Further, various metal oxides are used for a variety of applications. For example, indium oxide is a well-known material and used for a material of a transparent electrode which is needed in a liquid crystal display or the like.
Some metal oxides have semiconductor characteristics. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like. Thin film transistors in which a channel formation region is formed of such a metal oxide having semiconductor characteristics are already known (Patent Documents 1 and 2).